Ziegler-Natta catalysts are generally composed of a catalyst support material, a transition metal component and one or more ligands that satisfy the valence of the metal. The transition metal component is typically a Group 4-8 transition metal, with titanium, zirconium, chromium or vanadium being commonly used. The transition metal is often provided as a metal halide, such as TiCl4. Ziegler-Natta catalysts are used to effectively promote the high yield polymerization of olefins. In the polymerization of olefins, the catalyst is used in combination with an organoaluminum cocatalyst.
When used to catalyze polymerization of propylene, a third component may be used in the catalyst. The third component is an electron donor used to control the stereoregularity of the polymer. It can be either incorporated into the catalyst during its synthesis (an internal donor), or it can be added to the polymerization reactor during the polymerization reaction (an external donor). In some reactions, both an internal donor and an external donor may be used. Aromatic esters, diethers, succinates, alkoxysilanes and hindered amines are examples of compounds that may be used in forming polypropylene.
One known support material used in some Ziegler-Natta catalysts is MgCl2. The MgCl2 material is sometimes complexed with ethanol (EtOH). The EtOH reacts with the transition metal halide, such as TiCl4, in preparing the catalyst.
Methods of producing MgCl2-xEtOH complexes, where x is the average number of EtOH molecules in the support material, are described in several patents. For example, U.S. Pat. No. 5,468,698 to Koskinen describes methods for preparing a MgCl2-xEtOH support material. A melt MgCl2-xEtOH complex (x=3.3 to 5.5) is sprayed into a heated chamber to form a particulate MgCl2-xEtOH material in which x=2.0 to 3.2. Koskinen does not describe the composition of any particular catalyst made using the support material.
Catalysts utilizing MgCl2-xEtOH supports are also described. For example, U.S. Pat. No. 4,829,034 to Iiskolan describes a Ziegler-Natta catalyst, and a method for making the catalyst, using a MgCl2-xEtOH support in which x is about 3. In Iiskolan, the support material is first contacted with an internal donor, such as D-i-BP. The support D-i-BP complex is then combined with TiCl4 to form the catalyst.
U.S. Pat. No. 6,020,279 to Uwai describes a method for making a Ziegler-Natta catalyst by producing a MgCl2-xEtOH support in which x=1.5 to 2.1 and the support has an average particle diameter of 91 μm. The support is combined with a titanium halide, such as TiCl4, and an electron donor for 10 minutes to 10 hours at 120° C. to 135° C. in the presence of an aliphatic solvent.
While a variety of Ziegler-Natta catalysts have been developed, due to the importance of olefin polymerizations there remains a need to develop catalysts having improved activity. Improving the activity of the catalyst leads to higher product yields and reduces the quantity of the catalyst required for the olefin polymerization reaction which reduces the catalyst cost and the amount of catalyst impurities in the polymer (reduced ash content) resulting in polymers with a better performance profile.
Regardless of the method used to produce the MgCl2 support, or even if such a support is used in a typically produced Ziegler-Natta catalyst, only the unique method of combining the three essential components of the Ziegler-Natta catalyst as taught in this invention will produce the unusually high activity, hydrogen response and stereo-regulating activity found in the current invention.